1. Field of the Invention
The present invention relates generally to a high-density optical recording medium system and, more particularly, to an apparatus and method for reproducing records for optical recording medium, capable of detecting and compensating for detrack, tilt and defocus of the optical recording medium.
2. Description of the Related Art
In general, examples of optionally and iteratively rewritable optical recording medium include rewritable compact disc (CD−RW) and rewritable digital versatile disc (DVD-RW, DVD-RAM, DVD+RW).
These rewritable optical discs, particularly, DVD-RAMs have signal tracks made up of lands and grooves and enable the tracking control of an empty disc on which no information signal is written. Recently, information signals are also written on the tracks of lands and grooves so as to enhance recording density. For this purpose, the recent optical pickup for writing and reading information signals uses the shorter wavelength of laser beam with an increased number of apertures formed in the object lens and thereby reduces the size of beam for writing/reading records.
In order to achieve higher recording density, such a rewritable high-density optical disc is designed to have a reduced distance between the signal tracks, i.e., the smaller signal track pitch.
For the rewritable discs, it is naturally impossible to perform a disc control and a recording operation in an empty disc in which no information is written. Thus disc tracks are formed in lands and grooves to write information on, and control information for random access and rotation control is separately recorded in the disc, so as to enable tracking control in the empty disc.
The control information is, as shown in FIG. 1, written on the header pre-formatted at the beginning position of each sector, or along the track in the wobbling profile. The term “wobbling” as used herein refers to recording the control information on the boundary of tracks in accord to variation of laser beam by supplying power of laser diodes with information for modulating a predetermined clock and applying the modulated clock to the disc, e.g., information about a desired position and the rotational speed of the disc.
In a DVD-RAM, the header preformatted at the beginning position of each sector includes four header fields, e.g., header 1 field, header 2 field, header 3 field and header 4 field. Each header field has variable frequency oscillator (VFO) areas for generating a reference clock to acquire bit synchronization of read channels. In the present invention, the VFO areas present in the respective header fields (header 1 field˜header 4 field) are called VFO1˜VFO4.
That is, VFO1 and VFO 3 areas are present in the header 1 field and the header 3 field, VFO2 and VFO4 areas being in the header 2 field and the header 4 field. The VFO1 and VFO3 areas are longer and more stable for signal detection than the VFO2 and VFO4 areas.
The four header fields are staggered with respect to each other from the track center. FIG. 1 shows an example of the header for the first sector in a track. Referring to FIG. 1, the track boundary of the user area in which data are actually written has a wobbling profile.
An optical record reproducing apparatus also performs tracking and focus controls with an optical pickup in writing and reading information.
That is, tracking control, e.g., tracking servo involves detection of tracking error signals from electrical signals generated in accordance to the beam trace status and driving a tracking actuator in the optical pickup based on the tracking error signals to move an object lens of the optical pickup in the radial direction, thereby changing the position of the beam to trace a desired track.
There are some cases where detrack occurs that the beam focus is deflected from the track center, even though no tracking error signal is detected. Detrack does not adversely affect the compact discs.
However, detrack has an adverse effect on the optical discs such as DVD-RAM where data writing and reading is enabled in both lands and grooves, because the track pitch is narrowed for purpose of high densification.
Due to a depth difference between lands and grooves, detrack may occur in the tracks of the grooves even when no detrack is detected in the tracks of the lands. Likewise, the tracks of the lands may have detrack while there is no detrack detected in the tracks of the grooves.
If detrack occurs, writing/reading data becomes harder because the beam is ready to shift to the adjacent track to cause a cross talk and clear data from the track.
In a case where the beam focus is deflected from the disc surface during a focus control, i.e., focus servo, which case will be referred to as “defocus” hereinafter, quality of data deteriorates in writing and reading the data and thereby the system operation becomes unstable.
The focus servo drives a focus actuator in the optical pickup to move the optical pickup up or down and make the beam in focus according to the turning and up-and-down motions of the optical disc. That is, the focus actuator drives the object lens for convergence of beam in the upward/downward direction, e.g., in a direction of the focus axis to maintain a constant distance between the object lens and the optical disc.
However, in the optical discs such as DVD-RAM where data can be written in both lands and grooves, the lands and grooves differ in the focus offset from each other due to a depth difference and cause defocus even when no focus error signal is detected.
That is, due to the depth difference between the lands and grooves, defocus may be detected in the tracks of the grooves even when no detrack occurs in the tracks of the lands. Likewise, the tracks of the lands may have detrack while detrack is not detected in the tracks of the grooves.
As the defocus status cannot be known only from the focus error signals in this case, jitter characteristic deteriorates and the bit error rate (BER) increases. Recording data in this state may result in change recording characteristics of lands and grooves and hence deterioration of data quality, which makes the system operation unstable.
During a resin extracting and hardening process in fabrication of the optical disc, distortion may take place in the optical disc and cause eccentricity even when a central aperture is perforated in optical disc. Also, deviation of the central aperture causes eccentricity although the tracks of the disc are accurately provided in the radial form with a defined pitch. Thus as the disc turns with eccentricity, the central axis of the motor is not in perfect accord with the center of the track.
It is thus hard to read out the signals of a desired track only. So, in the CD and DVD systems, a tracking servo is performed according to the standards established for the deflected quantity such that the beam always traces the desired track in spite of eccentricity.
It means, the tracking servo generates electrical signals corresponding to the beam trace status and moves the object lens or the optical pickup body in the radial direction based on the generated electrical signals, to change the position of the beam and make the beam trace the accurate track.
Meanwhile, the beam can be deflected from a desired track due to a tilt of the disc as well as the eccentricity. This results from a mechanic error occurring when the disc is set on a spindle motor. That is, the focusing direction is not in perpendicular relation with the tracking direction. This slant state of the disc is called “tilt”.
Tilt is not so significant for compact discs that have a large tilt margin due to their wide track pitch. The term “tilt margin” as used herein refers to a compensable quantity of tilt of the disc. However, with a growing need of densification of the optical appliances such as optical discs, especially in the DVD having the narrower track pitch, a slight tilt of the disc causes the beam to shift to the adjacent track due to a small radial tilt margin for the jitter. This “detrack” is unavoidable by the tracking servo only. That is, the tracking servo may mistake that the beam is tracing the accurate track even when the beam is shifted to the adjacent track due to tilt, while focusing on the center of the track.
This makes it impossible to write/read data in/from a desired track. Thus a double distortion occurs when reading the erroneously written data.
To cope with this problem, there has been suggested a method in which the tilt of the disc can be detected with a dedicated tilt sensor, e.g., a tilt light-receiving device in an optical pickup. However, the method is not so efficient with a large size of the set.